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Do you have to use a lot of cannabis to increase the risk of psychosis? Is the risk increased if you are just a casual user?

Professor Sir Robin Murray: Psychosis is a young person’s illness, so, for example, I am very unlikely to develop schizophrenia, not because of my well-rounded personality, but because I am through the age of risk for developing it. So first of all, young people are more at risk of psychosis anyway, but there is some evidence that the earlier you start the greater your risk of developing psychosis.

if you start when you are thirteen and your teachers find out, your grades go down and you may get expelled from school, and you begin to mix with a lot of other kids who have a lot of problems, you may get into other drugs. That may cause a sort of cycle of decline into other lifestyles that increase the risk of psychosis. But the more worrying possibility is that as the brain is still developing, if you throw a lot of cannabis or what we call THC, Tetrahydrocannabinol, at your synapses, at your neurons, nerve cells in the brain, you may actually slightly divert the development of the brain and you may cause some changes which may not revert to normal when you stop taking the cannabis.

You certainly don’t go psychotic if you have a couple of puffs at a weekend. All the evidence is, is you have to try quite hard. So cannabis is a bit like alcohol. The vast majority of people who use alcohol use it moderately, they don’t take it every day, they don’t take it in vast excess, but everybody knows if you drink a bottle of whisky or a bottle and a half of vodka every day, you are at risk of causing cirrhosis or becoming alcoholic.

It’s the same with cannabis, that if you take a few puffs occasionally at the weekend, or on special occasions, if you are at a party, or something like that, this is not going to make you develop psychosis.

But if you’re smoking every day, if you’re smoking, particularly if you are smoking on the way to school, you’re smoking at lunchtime, you’re smoking in the evening, you’re smoking skunk particularly, as the months go by as the years go by you tend to become increasingly odd. The classic picture is that one begins to become suspicious of one’s friends, fight with one’s friends, have arguments with one’s parents, one’s school or college grades go down, one finds oneself increasingly a loner, increasingly enmeshed in one’s internal thoughts, increasing paranoid ideas about other people, then seeing and hearing things and becoming detached from reality. But this is not something that happens overnight, it may take 5 or ten years and I should say, of course, this only happens to a minority

Some people are lucky, they don’t have a pre-disposition and therefore, actually they can drink a lot of alcohol and not become alcoholic, or they can take a lot of cannabis and not become psychotic. But other people, by bad luck have their genes, which interact with on the one hand alcohol, or on the other hand cannabis.